Microprocessor-based systems and related arts, increasingly need read-only memory elements which can be altered by electrical means, i.e. memory elements which, although being able to retain data written thereon for relatively long periods of time (several years), offer the possibility of erasing and rewriting (reprogramming) all or some of the data contained therein by electrical means and essentially without needing to remove the microcircuit containing them from the apparatus for subjecting it to erasing treatments (preceding an eventual, necessarily total reprogramming) contemplating irradiation as it was necessary with the read-only memory devices of the FAMOS type, from "Floating Gate Avalanche Metal Oxide Semiconductor".
Lately, the art has progressed to the point where a number of nonvolatile memory devices, electrically alterable, have been successfully produced. Microprocessors or systems incorporating such memory devices, equally known as EE-PROM from "Electrically Erasable - Programmable Read-Only Memory" or as EA-PROM from "Electrically Alterable - Programmable Read-Only Memory", offer the great advantage, with respect of the devices of the prior art, of allowing both the erasing and rewriting of single bytes or the erasing of all the stored data.
The memory cell, that is the basic integrated semiconductor structure of such devices, is the so-called FLOTOX cell, from "Floating Gate Tunnel Oxide", which is described in detail in an article entitled: "16-J-EE-PROM Relies on Tunnelling for Byte-Erasable Program Storage" by W. S. Johson et al., "Electronics" of Feb. 28, 1980, pages 113-117. In this article, the author describes a FLOTOX structure where a cell utilizing a polycrystalline silicon floating gate structure, has such a structure charged with electrons (or vacancies) through a suitable "window" providing a thin layer of oxide between said floating gate structure and the monocrystalline silicon in correspondence to the drain region, by a Fowler-Nordheim tunnelling mechanism. That is, the mechanism exploited for trapping the charge in a floating gate electrode is conduction by tunnel effect of electrons (or vacancies) through a thin oxide dielectric layer caused by sufficiently high electric fields, generally over at least 10 MV/cm.